1. Field of the Invention
This invention relates to compositions and articles suitable for use in thermal transfer imaging processes, and also relates to graphic articles comprising a graphic image formed using the inventive compositions and articles, and methods of making such graphic articles.
2. Related Art
Graphic articles, sometimes referred to as signage articles, are used in a wide variety of applications, both for informational and for decorative purposes. For example, retroreflective sheetings are an important medium used in manufacturing graphic articles such as license plates, road/street markings, validation stickers, and package labels. Decals are a medium used for automotive labels and decoration.
Images on graphic articles may be formed by thermal transfer of a color layer from a first substrate or carrier, usually a plastic film, to a second substrate surface, e.g., the retroreflective sheeting or a component thereof. Thermal transfer printing methods, such as hot stamp printing or thermal mass transfer printing, form an image by selective transfer of portions of the color layer from the first substrate to the second substrate. Images formed on graphic articles may be, for example, alphanumeric characters, bar codes, or graphics.
Alternatively, images may comprise preformed color layers on a decal, which are transferred by hot transfer lamination. It is known to form graphic patterns on substrates using transfer articles bearing predesignated designs. G.B. Patent No. 1,218,058 (Hurst et al.) discloses transfers with an adhesive layer applied to only those areas intended to be transferred to the substrate; U.S. Pat. No. 4,786,537 (Sasaki) and U.S. Pat. No. 4,919,994 (Incremona et al.) disclose transfer graphic articles wherein the graphic design is formed via imagewise differential properties within the transfer film itself. One problem with such approaches is that a large and varied inventory must be maintained in order to provide a variety of graphic patterns.
Formation of desired graphic images from continuous layers via thermal transfer processes is also well known. For example, thermal mass transfer articles, typically comprising a carrier, optionally a release layer, and a transferable color layer have been known for some time. The article is contacted to a desired substrate such that the color layer is in contact with the substrate and heat is applied in imagewise fashion to cause imagewise portions of the color layer to release from the carrier and adhere to the substrate.
Hot stamping foils comprising a carrier, one or more color layers, and an adherence layer have also been known for some time. Such films have been used to provide imagewise graphic patterns, e.g., alphanumeric or decorative legends, to substrates via imagewise application of heat and/or contact or pressure. In some embodiments, additional members such as release layers are used to facilitate desired performance. In some embodiments, so-called "texture layers" and/or "ticks", metal layers, etc. are used as well to yield desired appearance. Hot stamping foils are also sometimes called hot stamp tapes or thermal transfer tapes.
The color layer(s), adherence layer, and any other layers that are selectively applied to the substrate should split or fracture in desired manner in order for the applied graphic pattern to have a desired edge appearance. Some illustrative examples of previously known hot stamping foils are disclosed in U.S. Pat. No. 3,770,479 (Dunning) U.S. Pat. No. 3,953,635 (Dunning), and U.S. Pat. No. 4,084,032 (Pasersky). It has also been known to transfer graphic patterns using means in addition to or other than heat to achieve imagewise separation of imaging material from a carrier and adhesion to a substrate. For example, U.S. Pat. No. 3,834,925 (Matsumura et al.) discloses a transfer material that utilizes solvent action to achieve imagewise deposition.
An advantage of the foregoing techniques is that the transfer film may be made as a uniform sheet, i.e., with no specific latent image embodied therein. The applicator defines the graphic pattern by controlling the application process, e.g., imagewise application of heat and/or contact or pressure permits maintenance of a smaller inventory of thermal transfer element material.
One well known use of hot stamping foils is to print legends on vehicle identification plates. For example, license plates produced using hot stamping foils have been used in Austria, Australia, Finland, Germany, Ireland, Portugal, and Switzerland. One commercially available hot stamping foil currently used on license plates with polyvinyl chloride cover films is believed to comprise a polyester carrier, about 28 microns thick; a color layer based on acrylic resins such as polymethyl methacrylate and containing carbon black pigments, about 5 microns thick; and an acrylate-based adherence layer, about 5 microns thick. Examples of resins that are believed to have been used in adherence layers include polyvinyl alcohol copolymers, nitrocellulose, and methyl methacrylate/butyl methacrylate copolymers.
Recently improved retroreflective sheetings have been made available which have cover films made of olefin-based materials or polyurethane-based materials to improve certain performance. As disclosed in the aforementioned U.S. Pat. No. 4,896,943 (Tolliver et al.), olefin-based cover films, e.g., ethylene/acrylic acid copolymers, can provide superior properties including abrasion and dirt resistance. Many conventional hot stamping foils do not achieve good adherence to such cover sheets, however, resulting in graphic patterns having unsatisfactory durability and performance.
More recently, U.S. Pat. No. 5,393,950 (Caspari) discloses hot stamping foils well suited for use on retroreflective articles wherein the foils comprise a carrier, optionally a release control layer, a color layer, and an adherence layer wherein the adherence layer comprises, and may consist essentially of, a mixture of an ethylene copolymer dispersion and an acrylic dispersion.
Graphic articles having images formed by thermal transfer normally provide satisfactory print quality, legibility, and adhesion. However, many presently known thermal transfer color layer formulations are compatible with only a limited class of retroreflective sheeting layers, primarily those layers comprising polyvinyl chloride (PVC), acrylics and polyurethanes. Sheetings having polyvinyl butyral, ethylene/acrylic acid copolymer, or melamine/alkyd copolymer surface layers may be difficult to print upon by known thermal transfer methods. Furthermore, PVC is not environmentally desirable.
Retroreflective articles are typically provided with cover films (e.g., made of polymethylmethacrylate (PMMA), plasticized PVC, alkyd resins, acrylic resins, and the like) to improve retroreflective performance under wet conditions and to protect the retroreflective elements.
To provide improved durability, embossability, and abrasion resistance, improved retroreflective sheetings with substantially thermoplastic cover films were developed, for example aliphatic polyurethanes and ethylene/acrylic acid copolymers, that latter including ionomers. One of the problems with the newer substantially thermoplastic cover film materials is that in order to achieve satisfactory adhesion of thermal transfer color layers to such films, chemical and/or physical priming of the cover film may be needed. For example, U.S. Pat. No. 5,393,590 (Caspari) discloses a hot stamp foil having a novel adherence layer over the color layer that permits effective thermal transfer printing upon polyolefin- or polyurethane-based surfaces.
Color layers of hot stamp foils and thermal transfer ribbons generally have low cohesive strength, which promotes efficient and desired imagewise transfer of color layer material from the first substrate to the second substrate surface during thermal transfer. However, images formed from such color layers should exhibit sufficient durability for many end uses. To improve the durability of the image, a protective layer often is formed thereover, e.g., by incorporating a cover layer in a hot stamp foil to be transferred along with the color layer, or by applying a clear coat over the transferred color layer.
U.S. Pat. No. 5,468,532 (Ho et al.), and a continuation-in-part thereof, U.S. patent application Ser. No. 08/506,926, filed Jul. 26, 1995, (Phillips), now pending, both incorporated herein by reference, disclose multilayer graphic articles comprising a substrate, a color layer disposed on the substrate, and a transparent, protective layer that overlies both the color layer and the substrate. The color layer is formed from ink formulations comprising a color agent and a copolymeric binder formed from the copolymerization product of an olefinic monomer and a second monomer having a pendant carboxyl group. The binder may be crosslinked through an ionic bond, a covalent bond, etc. In each case where the binder is crosslinked, carboxylic acid groups pendant from the copolymeric binder participate in the crosslinking reaction.
Although the above work is impressive, there still exists a need for thermoplastic thermal transfer compositions and thermal transfer articles that allow the thermoplastic composition to be readily and easily applied to a variety of substrates without using chemical and/or physical priming of the substrate, and which produce durable, weatherable images. Durability and weatherability of the compositions after being transferred to the substrate, with or without a cover layer, is also a primary goal.